By Josephine Mason and Hallie Gu

BEIJING (Reuters) – China, home to the world’s largest pig herd, reported its fourth outbreak of the deadly African swine fever on Thursday, with more than 400 pigs infected as the disease entered a fourth province, fanning worries about its rapid spread.

The infection killed 340 hogs on three farms in the coastal city of Wenzhou in eastern Zhejiang province, the Ministry of Agriculture and Rural Affairs said in a statement.

Zhejiang is the fourth region to discover the infection and the furthest from the original outbreak in Shenyang, the capital of northeastern Liaoning province.

Authorities have already culled more than 20,000 animals in an attempt to halt the spread of the highly contagious disease, which was first reported in China just three weeks ago.

But the vast distance between new cases and the original outbreak will escalate worries about how to control the disease across China’s vast hog herd.

To reach Wenzhou from Shenyang, it’s a 2,150 km (1,335 mile) drive south through the pig producing provinces of Hebei and Shandong as well as Jiangsu, another infected region.

The third infection detected at the weekend was just 890 km (553 miles) to the north in another coastal city, Lianyungang. The other was in land in central Henan province.

That may stoke concerns that the disease is being transmitted along busy trade routes that take pigs from market and farm in the northeast to slaughter and processing in the south. Some farmers have called a nationwide ban on transporting live pigs.

The rapid expansion of pig farms in recent years in China’s north-eastern cornbelt has increased the number of pigs being moved across country.

Local authorities in Wenzhou have banned the movement of live hogs, related products and animals that are easily infected both into and outside the affected area, the ministry said.

(Reporting by Josephine Mason, Hallie Gu and Beijing Monitoring Desk; Editing by Richard Pullin)